1. Field of the Invention
The present invention pertains to an economically viable synthesis of silsesquioxane anions and their derivatives, and to the use thereof.
2. Background Art
Silsesquioxane anions, particularly the cubeoctameric Si8O208− anion, are known. These polyhedral anions, having the general formula SinO5n/2n−, may be prepared by hydrolyzing organosilicates such as tetraethoxysilane (tetraethylsilicate) in the presence of a quaternary ammonium compound such as tetramethylammonium hydroxide, as described in D. Hoebbel et al., ZEITSCHRIFT FÜR ANORGANISCHE UND ALLGEMEINE CHEMIE, 384, pp. 43-52 (1971); I. Hasegawa et al., JOURNAL OF MOLECULAR LIQUIDS, 34, pp. 307-315 (1987); and Weidner et al. U.S. Pat. No. 5,047,492. Synthesis from fumed silica or precipitated silica employing quaternary ammonium compounds in aqueous medium is also known, as also disclosed in U.S. Pat. No. 5,047,492.
The silsesquioxane anions can serve as useful precursors to numerous polyhedral silsesquioxanes bearing reactive and/or non-reactive functional groups. Examples are octakis[vinyldimethylsiloxy]octasilsesquioxane, octakis[hydridodimethylsiloxy]octasilsesquioxane, and silsesquioxanes with mixed hydrido and vinyldimethylsiloxy functionalities which can serve as useful monomers in polymer synthesis, and as nuclei for a variety of functionalized “star” compounds or “dendrimeric” compounds.
Unfortunately, prior syntheses have been costly, as well as time and energy intensive. Organosilicates and fumed silicas, for example, generally employ tetrachlorosilane, SiCl4 or organohalosilanes as starting materials. SiCl4 and organohalosilanes, in turn, are prepared from metallic silicon which in turn is prepared by carbothermal reduction of SiO2. The expense of these starting materials precludes use of the silsesquioxane anions and derivatives prepared therefrom in all but the most demanding applications. Precipitated silica preparation is also energy intensive, involving reacting quartz sand with sodium carbonate at high temperature (1200° C.), dissolving the resulting sodium silicate in water, followed by neutralization with strong acid. Furthermore, preparation of silsequioxane anions from crystalline silica or low surface area silica is a time consuming reaction which is therefore capital and time intensive.
Silsesquioxane anions can be used to prepare functionalized silsesquioxanes. However, here too, the known processes are not economical. For example, ocktakis[trimethylsiloxy]octasilsesquioxane can be reacted in the presence of acid activated bleaching earth with 1,3-divinyl-1,1,3,3-tetramethyldisiloxane over long periods of time to prepare substituted silsesquioxanes bearing both trimethylsiloxy and vinyldimethylsiloxy groups. The product still bears non-functional trimethylsiloxy groups, even after 20 hours of reaction time, as indicated by Example 16 of U.S. Pat. No. 5,047,492. Other acidic catalysts such as acid (cationic) ion exchange resins have not been investigated, since octakis[trimethylsiloxy]octasilsesquioxane is known to suffer cleavage of the silsesquioxane cage, as disclosed by Hasegawaa et al., J. ORGANOMETALLIC CHEM., 441, p. 373 (1992), in which the above compound was exposed to Amberlyst® 15 cation exchange resin in heptane solution.
It would be desirable to provide a synthesis of polyhedral silsesquioxane anions, particularly the Si8O208− anion, by methods which employ inexpensive starting materials and synthetic processes. It would be further desirable to provide processes for synthesizing functionalized derivatives of such compounds at reasonable cost.